Polymer materials composed of organic substances are prone to deterioration by UV light in sunlight, such as decrease in mechanical strength and discoloration. Polymer materials containing highly reactive atoms such as nitrogen or sulfur atoms, aromatic rings, or branched structures, are particularly prone to such deterioration. Such polymer materials prone to photo-deterioration often contain additives such as various UV absorbers, hindered amine light stabilizers, and antioxidants. For example, for production of plastic eyeglass lenses having high refractive indices, UV absorbers for preventing photo-deterioration are often added to polymerizable monomer materials, and the resulting compositions are cured to produce the lenses.
Conventional optical materials containing UV absorbers, such as eyeglass lenses, absorb UV light up to 370 nm or 380 nm, as exemplified by plastic lenses disclosed in JP-60-51706-A. Properties of such plastic lenses were satisfactory when UV dose was so small that its adverse effects on human body were not necessarily considered. However, with the recent increase in UV dose caused by environmentally problematic destruction of the ozone layer, adverse effects of UV light on eyes have become a considerable issue, and the link between UV irradiation and keratitis or cataract has been requiring attention.
JP-1-230003-A proposes, in order to suppress the adverse effects of UV light, a lens containing a general-purpose UV absorber having low absorptivity in the long wavelength band such as 380-400 nm, in the amount of 0.05 to 0.2 wt % of the starting material monomers. This lens does not have sufficient absorption of long wavelength UV lights.
JP-9-269401-A proposes a method of impregnating a lens surface with a general purpose UV absorber similar to the above. This method, however, gives rise to new problems such as decrease in subsequent workability due to the modification of the lens surface material, yellowing of the lens surface, and limitation on the amount of the UV absorber with which to impregnate the lens surface. JP-11-271501-A proposes a method wherein a UV absorber having a low molecular weight of not higher than 360 is used in the amount of 0.7 to 5 wt % of the starting material monomers. With the lens obtained by this method, however, the low-molecular-weight UV absorber may migrate to the lens surface and transpire therefrom in long time wearing, which leads to decline in the UV absorptivity, yellowing, or generation of macule on the lens surface. Further, when the lens is provided with a hard coating, the coating may be peeled off. Even with such a special UV absorber, increase in its content will impair visible light beam transmittance due to the limited solubility of the UV absorber.
The conventional UV absorbers used in the above optical materials also disadvantageously yellow the lenses when added in a large amount for improving their effect, and thus are not suitable for use in producing colorless, transparent lenses. For suppressing such yellowing of lenses, JP-5-195445-A proposes a lens bluing method. This bluing method, however, encounters new problems such as declines in optical properties, UV absorptivity, and impact resistance, depending on the dyes or pigments used for bluing, the plastic materials for the lenses, or the kind and amount of the UV absorber. Thus development of the lenses is desired in which these properties are well balanced.
Incidentally, a variety of materials are conventionally known as plastic materials for use in optical materials, including, for example, diethylene glycol bisallyl carbonate resins, diallyl phthalate resins, thiourethane resins composed of thiol and isocyanate, acryl urethane, and halogenated di(meth)acrylate resins.